Resistance of the hippocampus in the lactating rat to N-methyl-D-aspartate (NMDA)-mediated excitation is not due to a nonfunctional receptor system

Regulation of anxiety during the postpartum period

Healthy mother-infant interactions are critical for the physical, cognitive, and psychological development of offspring. Such interactions rely on numerous factors, including a positive maternal emotional state. However, many postpartum women experience emotional dysregulation, often involving elevated anxiety. Neuroendocrine factors contributing to the onset of postpartum anxiety symptoms are mostly unknown, but irregularities in hypothalamic-pituitary-adrenal axis function, reduced prolactin and oxytocin signaling, or parturitional withdrawal of ovarian, placental and neural steroids could contribute to anxiety in susceptible women. Although the causes of initial onset are unclear, postpartum anxiety can be mitigated by recent contact with infants. Numerous neurochemical systems, including oxytocin, prolactin, GABA, and norepinephrine mediate this anxiolytic effect of infant contact. Insight into the etiology of postpartum anxiety disorders, and how contact with infants helps counter existing anxiety dysregulation, will surely facilitate the diagnosis and treatment of postpartum women at risk for, or experiencing, an anxiety disorder.

Orexin neurons express a functional pancreatic polypeptide Y4 receptor

The receptor subtypes that mediate the effects of neuropeptide Y (NPY) on food intake have not been clearly defined. The NPY Y4 receptor has been identified recently as a potential mediator of the regulation of food intake. The purpose of the present study was to characterize the central site of action of the Y4 receptor using a combination of neuroanatomical and physiological approaches. Using immunocytochemistry, Y4-like immunoreactivity was found to be colocalized with orexin cell bodies in the lateral hypothalamic area (LHA) and orexin fibers throughout the brain. In situ hybridization confirmed the expression of Y4 mRNA in orexin neurons. To determine the functional interaction between Y4 receptors and orexin neurons, we examined the effects of rat pancreatic polypeptide (rPP), a Y4-selective ligand, or NPY, a nonselective ligand, administered directly into the LHA on the stimulation of food and water intake and c-Fos expression. Both rPP and NPY significantly increased food and water intake when they were administered into the LHA, although NPY was a more potent stimulator of food intake. Furthermore, both NPY and rPP significantly stimulated c-Fos expression in the LHA. However, whereas rPP stimulated c-Fos expression in orexin neurons, NPY did not. Neither rPP nor NPY stimulated c-Fos in melanin-concentrating hormone neurons, but both activated neurons of an unknown phenotype in the LHA. These results suggest that a functional Y4 receptor is expressed on orexin neurons and that these neurons are activated in response to a ligand with high affinity for the Y4 receptor (rPP). Although these data suggest a role for central Y4 receptors, the endogenous ligand for this receptor has yet to be clearly established.

Integration of the regulation of reproductive function and energy balance: lactation as a model

Lactation is a physiological model for studying how the hypothalamus integrates peripheral signals, such as sensory signals (suckling stimulus) and those denoting energy balance (leptin), to alter hypothalamic function regulating food intake/energy balance and reproduction. The characteristics of food intake/energy balance during lactation are extreme hyperphagia, coupled with negative energy balance. The arcuate nucleus Neuropeptide Y (ARH-NPY) system is activated by: (1) brainstem projections specifically activated by the suckling stimulus, and (2) the decrease in leptin in response to the metabolic drain of milk production. NPY neurons from the ARH make direct contact with GnRH neurons and with CRH neurons in the PVH. NPY neurons also make contact with orexin and MCH neurons in the LHA, which, in turn, make contacts with GnRH neurons. Thus, the ARH-NPY system provides a neuroanatomical framework by which to integrate changes in food intake/energy with the regulation of cyclic reproductive function.

Stimulus-dependence of NO-mediated neuronal activation: a Fos expression study in the hippocampus and cortex of lactating rats

We investigated how simultaneous manipulation of suckling and i.c.v. injection of nitric oxide (NO)-related pharmacological agents affect Fos activation in the hippocampus and cortex of lactating rats. Dams received drugs or vehicle injection just prior to reunion with their pups following a 12 h separation and Fos protein activation was examined after 2 h of suckling. In conscious dams, sodium nitroprusside (SNP, an NO donor) significantly activated Fos expression in the hippocampus and cortex compared with vehicle or an NO synthase inhibitor. Urethane anaesthesia abolished Fos expression in the cortex, but not in the hippocampus following simultaneous SNP injection and resumption of suckling. Fos activation in the hippocampus and cortex was significantly attenuated when SNP was injected alone (without dam-pups reunion after separation). Coincident injection of SNP with a NO synthase inhibitor abolished all Fos activation in the hippocampus and cortex. In chronically lactating rats (without any interruption of suckling) SNP injection failed to activate any Fos expression. These observations underscore the importance of stimulus modality in revealing the dynamics of NO-mediated neuronal function within a discrete tissue entity such as the hippocampus or the cortex, which may be relevant to other regions of the brain.